build: update to Rust edition 2021

This enables feature resolver version 2
so dev-dependencies features
are not enabled for non-dev builds.

Cargo.toml

@@ -9,7 +9,7 @@ license = "MIT OR Apache-2.0"
 authors = ["dignifiedquire <me@dignifiedquire.com>"]
 categories = ["email"]
 keywords = ["email", "smtp", "mailer"]
-edition = "2018"
+edition = "2021"
 
 [badges]
 maintenance = { status = "actively-developed" }